Patent GB-1467845-A (GB)
Title: Television receiver
IPC: H03D1/22
Filed: 1974-05-03
Abstract: 1467845 Synchronous detectors PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd 3 May 1974 [8 May 1973] 19586/74 Headings H3Q and H3T In a television receiver having a synchronous detector the regenerated carrier for the detector is derived from the incoming signal when the receiver is well off tune, but upon approaching the correct tuning point the carrier regenerator is automatically switched to become selfoscillatory to generate locked carrier waves. Frequency pulling during tuning is thus avoided. The receiver comprises a tuner 1, Fig. 1, including a mixer fed by local oscillator 7 and I.F. circuits 13 feeding (1) synchronous detector 19, (2) carrier regenerator 25 and (3) phase detector 61. With the receiver well off tune, electronic switch 45 is in the position shown and feeds I.F. signal to narrow-band filter 53 which supplies a carrier signal to (1) the carrier input 21 of synchronous detector 19, (2) amplifier 49, and (3) input 59 of phase-detector 61, the second input 63 of which is derived from the output of I.F. amplifier 13. When the receiver is nearly in tune, filter 33 tuned to the intercarrier sound frequency gives a sufficient output for the output from diode rectifier 39 to cause reversal of switch 45: amplifier 49 and narrow-band filter 53 then form a regenerative feedback loop generating oscillations which are locked to the signal carrier frequency by means of phase detector 61 which controls the frequency to which filter 53 is tuned. In a second embodiment the phase detector for detecting the on-tune condition comprises transistors 225-229, 231, 233 being fed with balanced signals from I.F. transformer 203 via emitter-followers 205, 207 and with a comparison signal from emitter-follower 243, via 90 degrees phase-shifter 237. Emitter-followers 205, 207 also feed signal inputs 213, 215 of synchronous detector 217 and into emitter-coupled pair 221, 223 of the carrier regeneration circuit: the pair 221, 223 feed sharply-tuned circuit 279 from which carrier is supplied to inputs 283, 285 of detector 217 via emitter-followers 241, 275. The phase detector 225-229, 231, 233 provides two D.C. outputs which are fed via emitter-followers 245, 247 into a common resistor 253: the voltage across this is a minimum at exact tune and rises on either side of tune, rendering transistor 255 conductive and transistor 257 conductive, so that transistors 221, 223 are conductive and pass signal, but transistors 271, 273 non- conductive. Approaching the tuning point, the drop in base bias renders transistor 255 non- conductive, cutting off transistors 221, 223; transistor 257 then becomes conductive, rendering transistors 271, 273 conductive, to form with emitter-followers 241, 275 a regenerative circuit oscillating at the required carrier frequency. The frequency and phase is controlled by transistors 249, 251 which develop an additional A.C. voltage across tuned circuit 281, the A.C. voltage being derived from a transistor 281 in the common emitter circuit of transistors 249, 251, which in turn obtains its input from the 90 degree phase-shifter 237. The extra A.C. voltage depends on the amplitude and polarity of the D.C. voltage difference at the bases of transistor 249, 251, which comes from the phase detector 225-229, 231, 233.